In conventional systems, a superconducting magnet becomes superconductive when placed in an extremely cold environment, such as by enclosing the magnet in a cryostat or pressure vessel and surrounding the magnet with a liquid cryogen such as liquid helium for cooling.
The zero boil-off 4.2K magnet cooling system briefly consists of vacuum insulated liquid helium reservoir integrated into a cryostat with a radiation shield with temperature having a range of 40K to 70K and a liquid helium recondencing system in which saturated helium gas is recondenced into liquid causing the reservoir to be at saturated operational condition around 4.2K. The helium reservoir is subjected to a heat load caused by a combination of heat conducted through the structural supports and radiation from its surroundings. Recondencing systems must be able to overcome the applied heat load in order to achieve saturated equilibrium conditions. The main component of the recondencing system is the cryocooler, which is the heat sink or the heat removal source of this system thru a heat exchanger in thermal contact with the cryogen. The current helium zero boil off system is based on a cryocooler design with an excess of cooling capacity to overcome variable heat load condition due to component manufacturing process variations, performance degradation due to component orientation and to be able to satisfy heat loads requirement at the end of the component life.
The conventional cryocooler during normal steady state (pressure and temperature) operational condition removes heat from the liquid helium reservoir and radiation shield (for two stage systems) at a constant rate to keep gas recondencing and therefore maintaining the liquid helium at a saturated condition. Typical cryocooler selection and use is driven by the maximum cooling capacity of the component at given temperature that is achieved thru an optimal constant oscillating frequency of the gas flow (thru constant cryocooler displacer/piston strokes frequency). High oscillation rates designed for maximum cooling capacity increase wear on the sealing components of the cryocooler leading to a reduction in component life. Magnets manufacturing component variations and component cool down (transients) demands high heat load removal by the cooler (maximum cooling capacity operational condition) but as the system (environment, radiation shield, cryocooler and cryogen reservoir) achieves steady state, the cooling capacity requirements are reduced. Since the cooling capacity of the cryocooler can not be change, the excess cryocooler cooling capacity is balanced by additional heat provided by electric heaters in the cryogen pressure vessel which is activate by low and high setting pressure limits creating pressure oscillation in the pressure vessel.
Large pressure variation in the cryogen pressure vessel can produce oscillating stresses in the superconducting magnet coils affecting the homogeneity of the magnetic field and therefore affecting image quality for MRI superconducting magnets. The invention will provide a more stable pressure environment for superconducting MRI magnet systems.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for variable heat load capacity cryocooler. There is also a need for an improved method and apparatus for controlling a cryocooler in a zero boiloff cryogen cooled recondensing superconducting magnet assembly including superconducting magnet coils suitable for magnetic resonance imaging.